1) Filed of the Invention
The present invention relates to a semiconductor laser module that is utilized in an optical transmitter. Particularly, the invention relates to a wavelength monitor inside an optical signal transmission module that is utilized in a wavelength division multiplexing (WDM) system, and a laser module with integrated wavelength monitor.
2) Description of the Related Art
A semiconductor laser device can obtain large laser output power when an injection current is increased. In general, the calorific value of the device itself increases in proportion to the injection current. The increase in heat affects the characteristics of semiconductor layers and optical parts that constitute the semiconductor laser device. The increase in heat generates various inconveniences. For example, the wavelength of an actual laser output is deviated from a desired wavelength, and the life of the device is shortened.
Particularly, in the semiconductor laser device that is used in a high-density WDM system, it is necessary to carry out wavelength control precisely. Therefore, it is necessary that the wavelength of an optical signal is stable over a long period of time. For this purpose, there has been developed a technique of providing a wavelength monitoring function inside a laser module that is built in with a semiconductor laser device.
FIG. 15 is a top plan cross-sectional view of a laser module that has been proposed by the applicant of the present invention in U.S. patent application No. 10/032,612 (a first conventional example). In a laser module 200 shown in FIG. 15, the front end of an optical fiber 11 is fixed to a package 201 with a ferrule 12, in order to emit a laser beam generated by a semiconductor laser device 20 into the optical fiber 11.
On the bottom surface of the package 201, there are adjacently disposed a first thermo-module and a second thermo-module not shown that can be heated or cooled based on a control from the outside. A base 30 made of CuW or the like is mounted on the first thermo-module. On this base 30, there are disposed a sub-mount 34 that is mounted with the semiconductor laser device 20 and a thermistor 21 that measures the temperature of the semiconductor laser device 20, a condenser lens 33 that connects a laser beam output from a front end surface of the semiconductor laser device 20 to the optical fiber 11, an optical isolator 32 that interrupts a return light reflected from the optical fiber 11, and a parallel lens 35 that makes parallel a monitoring laser beam that is output from a back end surface of the semiconductor laser device 20. Sections including the base 30, the condensing lens 33, the sub-mount 34, and the parallel lens 35 will be collectively called a laser section.
On the other hand, a base 50 made of CuW or the like is mounted on the second thermo-module. On this base 50, there are disposed a prism 51 that divides a monitoring laser beam that is output from the back end surface of the semiconductor laser device 20, into two directions at a predetermined angle, an optical filter 52 into which one of the lights divided by the prism 51 is incident, and a sub-mount 53. On the front surface (the surface of a laser emission direction) of the sub-mount 53, there are disposed on the same plane a first optical detector 41 that receives the other light divided by the prism 51, and a second optical detector 42 that receives the light that has been transmitted through the optical filter 52. Photodiodes are used for the first optical detector 41 and the second optical detector 42. The prism 51 is constructed of light incident surfaces 51a and 51b mutually formed at a predetermined angle to which the monitoring beam is incident, and a light emission surface 51c from which lights that have been divided within the prism 51 are emitted.
In the vicinity of the portion at which the prism 51 is fixed, there is provided a thermistor 54 that monitors the temperature of the optical filter 52. The base 50 and sections including the various constituent elements provided on the base 50 will be collectively called a wavelength monitor.
Based on the above structure, the laser module 200 controls the temperatures of the first thermo-module and the second thermo-module, thereby to realize a stable laser oscillation. The temperature control carried out by this laser module 200 will be briefly explained below. First, the monitoring laser beam that is output from the back end surface of these miconductor laser device 20 passes through the parallel lens 35, and is divided into two directions by the prism 51.
One of the lights obtained by dividing by the prism 51 is converted into a current by the first optical detector 41, and this current is converted into a voltage by a current-voltage converter not shown. This voltage is used as a reference voltage. The other light obtained by the dividing by the prism 51 passes through the optical filter 52, and is converted into a current by the second optical detector 42, and this current is converted into a voltage by a current-voltage converter not shown. This voltage is used as a signal voltage. The optical filter 52 has characteristics of different transmittances for the wavelengths of the incident light. This optical filter 52 is formed with an etalon, for example. A difference between the signal voltage obtained by passing the light of a desired wavelength through the optical filter 52 and the reference voltage will be called a reference voltage difference. Then, it is possible to know a wavelength deviation by comparing a voltage difference between the actual reference voltage and the signal voltage with the reference voltage difference.
This wavelength deviation is due to the heating of the semiconductor laser device 20. Therefore, in order to correct this deviation, the sub-mount 34 beneath the semiconductor laser device 20 may be cooled. The voltage that shows the wavelength deviation that is obtained based on the above comparison is used as a control voltage for a controller not shown to control the temperature of the first thermo-module disposed beneath the base 30. The first thermo-module is operated as a cooler. With this arrangement, the semiconductor laser device 20 is cooled via the first thermo-module, the base 30, and the sub-mount 34, and is feedback controlled to output the laser beam of a desired wavelength. This will hereinafter be referred to as a wavelength locking. When excessive cooling is obtained based on the feedback control, the first thermo-module operates as a heater.
The characteristic of the optical filter 52 that is formed with etalon changes depending on the temperature. Therefore, it is preferable to keep constant the temperature of the optical filter 52. The controller not shown calculates a difference between a desired temperature and the temperature detected by the thermistor 54, and controls the temperature of the second thermo-module disposed beneath the base 50, by using the voltage corresponding to this difference as a control voltage. With this arrangement, the optical filter 52 is heated or cooled via the second thermo-module and the base 50, and is stabilized at the desired temperature.
FIG. 16 is a top plan cross-sectional view of a laser module which shows a second conventional example. In FIG. 16, sections that are common to those shown in FIG. 15 are attached with identical reference symbols, and explanation of these sections will be omitted. A laser module 210 shown in FIG. 16 is different from the laser module 200 shown in FIG. 15 in only the structure of the wavelength monitor.
Specifically, on abase 50, there are disposed sub-mounts 61 and 62 that are separated from each other so that their main surfaces form a right angle, a half-mirror 71 that transmits a monitoring laser beam output from the back end surface of a semiconductor laser device 20 to a sub-mount 61 and that also reflects the monitoring laser beam to a sub-mount 62, and an optical filter 72 to which the light reflected from the half-mirror 71 is incident. On the front surface (main surface) of the sub-mount 61, there is provided a first optical detector 63 that receives the light that has transmitted through the half-mirror 71. On the front surface (main surface) of the sub-mount 62, there is provided a second optical detector 64 that receives the light that has transmitted through the half-mirror 72. The laser module 210 carries out the temperature control in a similar manner to that of the laser module 200.
However, according to the laser modules that make it possible to carry out the wavelength locking in the first and second conventional examples, a stray light of laser beams occurs in the wavelength monitor. Therefore, it has not been possible to carry out a precise wavelength locking. This problem will be explained below.
FIG. 17 is an explanatory view which explains the problems of the first conventional example, and this is an enlarged view of the wavelength monitor shown in FIG. 15. In FIG. 17, a monitoring laser beam that has been output from the back end surface of the semiconductor laser device 20 is incident to the prism 51 via the parallel lens 35. The laser beam that has been incident to the light incident surfaces 51a and 51b of the prism 51 is divided into a light 82 of an emission angle xcex81 and a light 83 of an emission angle xcex82 (=xcex81) relative to a center line 81 determined according to the shape of the prism 51.
The light 82 is incident straight to the first optical detector 41, and the light 83 is incident to the optical filter 52. Apart of the light 83 that has been incident to the optical filter 52 is transmitted through the optical filter 52 and reaches the second optical detector 42, and the reset of the light 83 is reflected by the front surface of the optical filter 52. A reflection light 84 reaches the front surface of the emission surface 51c of the prism 51, and is further reflected as a reflection light 85. The reflection light 85 is substantially equal to the route of the light 82, and therefore, reaches the detection range of the first optical detector 41.
In other words, the first optical detector 41 receives the reflection light 85 as a stray light, in addition to the light 82 divided by the prism 51. Consequently, the output current varies. When this variation occurs, the reference voltage becomes inaccurate, and the wavelength locking becomes unstable. The first optical detector 41 is used not only to generate the reference voltage of the wavelength locking but is also used as a power monitor to control the injection current of the semiconductor laser device 20. Therefore, the above variation causes the interference of the stability of the output power of the semiconductor laser device 20.
According to the laser module 200 in the first conventional example, one component of the reflection light 84 is transmitted through the prism 51, and reaches the semiconductor laser device 20. This may give a bad influence to the oscillation operation of the semiconductor laser device 20.
FIG. 18 is an explanatory view which explains the problems of the second conventional example, and this is an enlarged view of the wavelength monitor shown in FIG. 16. In FIG. 18, a monitoring laser beam that has been output from the back end surface of the semiconductor laser device 20 is incident to the half-mirror 71 via the parallel lens 35. The laser beam that has been incident to the half-mirror 71 is divided into the transmission light 85 and the reflection light 86 according to the disposition of the half-mirror 71 (the inclination in the lateral direction of the main surface relative to the incident direction).
The transmission light 85 is incident straight to the first optical detector 63, and the reflection light 86 is incident to the optical filter 72. A part of the reflection light 86 that has been incident to the optical filter 72 is transmitted through the optical filter 72 and reaches the second optical detector 64, and the reset of the reflection light 86 is reflected by the front surface of the optical filter 72. A reflection light 87 that has been reflected from the front surface of the optical filter 72 reaches the front surface of the half-mirror 71, and is further reflected as a reflection light 88. The reflection light 88 is substantially equal to the route of the laser beam that has been output from the back end surface of the semiconductor laser device 20, and therefore, the reflection light 88 reaches the semiconductor laser device 20. Consequently, this gives a bad influence to the oscillation operation of the semiconductor laser device 20.
It is an object of the present invention to provide a wavelength monitor capable of reducing the influence of a reflection light that is generated in an optical filter that constitutes the wavelength monitor, and capable of obtaining an optical detection signal of high quality, and a laser module with integrated wavelength monitor.
According to one aspect of the present invention, there is provided a wavelength monitor that detects a change in the wavelength of a laser beam, the wavelength monitor comprising: an optical dividing unit that divides the laser beam into a first light and a second light; a first optical detecting unit that detects the intensity of the first light; an optical filtering unit that has a wavelength characteristic and has a light incident surface disposed at a predetermined angle from a surface perpendicular to the optical axis of the second light so as to transmit a part of the second light through the light incident surface and make the rest of the second light incident into the light incident surface; and a second optical detecting unit that detects the intensity of the light that has been transmitted through the optical filtering unit, wherein the predetermined angle is an angle that has been adjusted such that the other part of the second light that proceeds after being reflected from the light incident surface of the optical filtering unit and further reflected from the light emission surface of the optical dividing unit is not connected to the first optical detecting unit.
According to the above aspect of the invention, the light that has been reflected from the front surface of the optical filtering unit is incident to the optical dividing unit through a route different from the incident route of the second light. Therefore, it is possible to make the route of the reflection light that has been generated by a further reflection of the light from the front surface of the optical dividing unit deviate from the route that has been connected to the first optical detecting unit.
According to another aspect of the invention, there is provided a wavelength monitor that detects a change in the wavelength of a laser beam, the wavelength monitor comprising: an optical dividing unit that divides the laser beam into a first light and a second light; a first optical detecting unit that detects the intensity of the first light; an optical filtering unit that has a wavelength characteristic and has a light incident surface at an angle so as to transmit a part of the second light through the light incident surface and reflect the other portion of the second light from the light incident surface to an upper direction or a lower direction than the incident route of the second light; and a second optical detecting unit that detects the intensity of the light that has been transmitted through the optical filtering unit.
According to the above aspect of the invention, the light that has been reflected from the front surface of the optical filtering unit is incident to the optical dividing unit through a route different from the incident route of the second light. Therefore, it is possible to make the route of the reflection light that has been generated by a further reflection of the light from the front surface of the optical dividing unit deviate from the route of the first light that has been generated by dividing by the optical dividing unit.
According to still another aspect of the invention, there is provided a wavelength monitor that detects a change in the wavelength of a laser beam, the wavelength monitor comprising: an optical dividing unit that has a light incident surface and/or a light emission surface at an angle to divide the laser beam into a first light and a second light, and transmit the laser beam to an upper direction or a lower direction than the incident route of the laser beam; a first optical detecting unit that detects the intensity of the first light; an optical filtering unit that has a wavelength characteristic, and that transmits a part of the second light and reflects the other portion of the second light; and a second optical detecting unit that detects the intensity of the light that has been transmitted through the optical filtering unit.
According to the above aspect of the invention, the light incident surface or the light emission surface of the optical dividing unit is inclined to an upper direction or a lower direction. Therefore, it is possible to make the route of the first light that has been generated by dividing by the optical dividing unit deviate from the route of the light that has been reflected by the optical filtering unit and further reflected from the light emission surface of the optical dividing unit.
According to still another aspect of the invention, there is provided a wavelength monitor that detects a change in the wavelength of a laser beam, the wavelength monitor comprising: a prism that divides the laser beam into a first light and a second light, and has an asymmetrical shape relative to an incident direction of the laser beam as a cross-sectional shape, or has a cross-sectional shape disposed to be asymmetrical relative to the incident direction of the laser beam; a first optical detecting unit that detects the intensity of the first light; an optical filtering unit that has a wavelength characteristic, and that transmits a part of the second light and reflects the other portion of the second light; and a second optical detecting unit that detects the intensity of the light that has been transmitted through the optical filtering unit.
According to the above aspect of the invention, the cross-sectional surface of the prism is asymmetrical relative to the incident laser beam. Therefore, it is possible to make the route of the light that has been reflected by the optical filtering unit and further reflected from the light emission surface of the prism deviate from the route of the first light that has been divides by the prism.
According to still another aspect of the invention, there is provided a wavelength monitor that detects a change in the wavelength of a laser beam, the wavelength monitor comprising: an optical dividing unit that divides the laser beam into a first light and a second light; a first optical detecting unit that detects the intensity of the first light; an optical filtering unit that transmits a part of the second light; and a second optical detecting unit that detects the intensity of the light that has been transmitted through the optical filtering unit, wherein the inclination of a light incident surface and/or a light emission surface of the optical dividing unit or the optical filtering unit relative to the laser beam has been selected to prevent a stray light from the optical filtering unit to the first optical detecting unit.
According to the above aspect of the invention, the light reflected from the front surface of the optical filtering unit is incident to the optical dividing unit through the route that is different from the incident route of the second light. Therefore, it is possible to make the route of the reflection light that has been generated by a further reflection of the light from the surface of the optical dividing unit deviate from the route of the first light that has been divided by the optical dividing unit.
According to still another aspect of the invention, there are provided a laser module comprising a semiconductor laser device that outputs a laser beam, and the wavelength monitor.
According to the above aspect of the invention, it is possible to provide a laser module with integrated wavelength monitor.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.